1. Field of the Invention
The present invention relates to a method and apparatus for controlling the fuel cut-off in an internal combustion engine mounted on a four-wheel drive (4WD) vehicle.
2. Description of the Prior Art
In general, the fuel cut-off is activated to stop the injection of fuel during deceleration, thereby improving fuel consumption. The control of the fuel cut-off depends upon the opening of a throttle valve, the engine speed, and the like. For example, when the throttle valve is completely closed and the engine speed is higher than the required fuel cut-off engine speed, the fuel cut-off is activated. Contrary to this, when the throttle valve is not completely closed, or when the engine speed is lower than the required fuel cut-off recovery engine speed, the fuel cut-off is released. In this case, the fuel cut-off engine speed is higher than the fuel cut-off recovery engine speed, thereby obtaining the hysteresis characteristics of the engine speed. In addition, both the fuel cut-off engine speed and the fuel cut-off recovery engine speed are dependent upon engine state parameters such as the coolant temperature of the engine.
In a 4WD vehicle, a higher driving power is generally required during the 4WD mode than during the 2WD mode, and accordingly, the transmission gear ratio is lower during the 4WD mode than during the 2WD mode. As a result, even at the same vehicle speed, the engine speed during the 4WD mode is higher than the engine speed during the 2WD mode.
In the prior art, the fuel cut-off is activated under the same conditions, regardless of whether or not the vehicle is in the 4WD mode or the 2WD mode. That is, the fuel cut-off engine speed during the 4WD mode is the same as the fuel cut-off engine speed during the 2WD mode, and the fuel cut-off recovery engine speed during the 4WD mode is the same as the fuel cut-off recovery engine speed during the 2WD mode.
In the above-mentioned prior art, however, since the transmission gear ratio is lower during the 4WD mode than during the 2WD mode, the fuel cut-off is activated more frequently during the 4WD mode than during the 2WD mode. That is, the fuel cut-off operation and the fuel cut-off recovery operation are repeated frequently during the 4WD mode.
It should be noted that the vehicle is generally driven in the 4WD mode on a snow-covered road, a sandy road, a mountain road, an uneven-surface road, or the like, in which conditions it is difficult to drive in the 2WD mode. Therefore, such frequent repetition of the fuel cut-off and fuel cut-off recovery operations lowers the driving performance during the 4WD mode, compared with that during the 2WD mode, since a minus torque caused by the fuel cut-off, a plus torque caused by the fuel cut-off recovery, and the torque changes due to the minus and plus torques are transmitted to all four wheels.